AMD’s GPUs were much faster than Intel’s, and making GPUs for this kind of application was something AMD already did. Nvidia didn’t, so would have to design a whole chip from scratch, and didn’t really have a power efficiency advantage (in recent generations where AMD’s desktop cards have run hot, it’s because they’ve been clocked high to keep up with Nvidia’s cards, but the same architecture runs cool when clocked lower for mobile applications, e.g. Vega was notoriously inefficient on the desktop due to being delayed two years and having to compete with a different generation than it was designed to, but was great in laptop APUs). Intel would also have gained experience with chiplets and packaging a fast GPU with a CPU. It let everyone involved make more money than doing it any other way.
AMD’s GPUs were much faster than Intel’s, and making GPUs for this kind of application was something AMD already did. Nvidia didn’t, so would have to design a whole chip from scratch, and didn’t really have a power efficiency advantage (in recent generations where AMD’s desktop cards have run hot, it’s because they’ve been clocked high to keep up with Nvidia’s cards, but the same architecture runs cool when clocked lower for mobile applications, e.g. Vega was notoriously inefficient on the desktop due to being delayed two years and having to compete with a different generation than it was designed to, but was great in laptop APUs). Intel would also have gained experience with chiplets and packaging a fast GPU with a CPU. It let everyone involved make more money than doing it any other way.